Belgium has freed the sole suspect charged over last week's Islamic State attacks in Brussels due to a lack of evidence, raising fresh questions about the handling of the case by under-fire Belgian authorities.

Key points:

Sole charged suspect Faycal C is released due to unsubstantiated charges

Source close to investigation says Faycal C "not the man in the hat"

Terror probe widens as authorities struggle to connect the dots

Prosecutors charged the man identified as Faycal C on Saturday with offences including "terrorist murder", and had been investigating the theory that he was a third airport attacker —the so-called "man in the hat"—who fled when his bomb failed to go off.

His release comes as a new blow to an inquiry already dogged by accusations that Belgium missed a series of leads in cracking down on a jihadist network linked to the attacks and a similar assault in Paris in November.

The Belgian federal prosecutor's office said in a statement that "the indications that led to the arrest of Faycal C were not substantiated by the ongoing inquiry".

"As a result, the subject has been released by the examining magistrate."

A source close to the inquiry added: "Investigators have established that [Faycal] was not the 'man in the hat'."

As Belgium struggles to come to terms with the tragedy, recriminations continue over whether the authorities could have prevented it, as the links to the Paris attacks in which 130 people were killed become clearer by the day.